Google has had some success with it obviously, but I wonder how it would have worked had they done it by Quarter or semi-annually instead.
Say, you meet a certain threshold in your performance review, you get a lump sum of ~20% of the time period that the review covers.
Say for a quarterly review, if you meet the threshold, you get 2.6 weeks. Or semi-annually you get 5.2 weeks, straight. You have to be at the office, but you can work on your own project during that time.
5.2 weeks is some serious hacking time, enabling a great deal of extended focus. Highly productive Google engineers could probably bootstrap a startup MVP in that time, except of course Google would own it.